The present invention relates to a contact image sensor for sensing characters, diagrams and drawings on a document, and more particularly to a color contact image sensor which can sense and transmit characters, diagrams, and drawings of a colored document.
Generally, a contact image sensor (hereinafter referred to as "CIS") is utilized in a facsimile to sense the shapes and gray levels of characters, diagrams, and drawings on a document to be transmitted or in a hand scanner, a peripheral equipment of a personal computer, to sense the shapes and gray levels of the images on a document to be processed by the personal computer. However, the conventional CIS has a problem in that it can sense a black/white document, but not a colored document. This problem is described with reference to the attached drawings as follows.
With reference to FIG. 1, a CIS comprising m sensing portions B1 to Bm connected commonly to n data lines DL1 to DLn and respectively to m address lines AD1 to ADm is described. The m sensing portions B1 to Bm are composed of n dot cells DS11 to DSmn, each of which is composed of one of photodiodes PD11 to PDmn and one thin film transistor (hereinafter referred to as "TFT"). The anodes of the photodiodes PD11 to PDmn are commonly connected to a reverse bias voltage source 14 and the source terminals of the TFTs Q11 to Qmn are respectively connected to the cathodes of the photodiodes PD11 to PDmn. The drain terminals of the TFTs Q11 to Q1n within a first sensing portion B1 are respectively connected to n data lines DL1 to DLn, and the gate terminals of the TFTs Q11 to Q1n within the first sensing portion B1 are commonly connected to a first address line AD1. Also, the drain terminals of the TFTs Qm1 to Qmn within the m'th sensing portion Bm are respectively connected to n data lines DL1 to DLn, and the gates of the TFTs Qm1 to Qmn within the m'th sensing portion Bm are commonly connected to the m'th address line ADm.
The CIS additionally comprises a driving portion 10 connected to the first-to-m'th address lines AD1 to ADm, and a signal outputting portion 12 connected to the first-to-n'th data lines DL1 to DLn. The driving portion 10 sequentially drives the first-to m'th sensing portions B1 to Bm through the first-to-m'th address lines AD1 to ADm, thereby sequentially supplying the outputs of the first-to-m'th sensing portions B1 to Bm to the signal outputting portion 12. And, the signal outputting portion 12 transmits the sequentially received outputs of the first-to-m'th sensing portions B1 to Bm to a signal processing portion (not shown).
Meanwhile, the photodiodes PD11 to PDmn within the first-to-m'th sensing portions B1 to Bm generate a current corresponding to the optical density from an incident light source (not shown) reflected by a document, and store the generated current in the internal capacitor (not shown). And, the TFTs Q11 to Qmn transmit the current stored in the photodiodes PD11 to PDmn to the signal outputting portion 12, when a driving signal is supplied from the driving portion 10 to their gates. In detail, the n TFTs Q11 to Q1n of the first sensing portion B1 transmit the current stored in the photodiodes PD11 to PD1n to the signal outputting portion 12, when a driving signal is supplied from the driving portion 10 through the first address line AD1. And, the n TFTs Qm1 to Qmn of the m'th sensing portion Bm transmit the current stored in the photodiodes PDm1 to PDmn to the signal outputting portion 12, when a driving signal is supplied from the driving portion 10 through the m'th address line ADm. Then, the signal outputting portion 12 converts the current generated by the photodiodes PD11 to PDmn into digital data. The digital data has different logic values according to the amount of current generated by the photodiodes PD11 to PDmn. The amount of current generated in the photodiodes PD11 to PDmn varies according to the gray levels of the characters, diagrams, and drawings in a document.
FIG. 2 shows a layout of the first-to-third dot cells DS11 to DS13 of the first sensing portion B1 which is a part of CIS. With reference to FIGS. 1 and 2, the first-to-third photodiodes PD11 to PD13, whose anodes are connected to the reverse bias voltage line 14, are horizontally arranged in parallel. The first-to-third TFTs Q11 to Q13, whose source electrodes are connected to the cathodes of the diodes PD11 to PD13, respectively, are arranged in parallel with the first-to-third photodiodes PD11 to PD13. For the first-to third photodiodes PD11 to PD13 to maintain a relatively large light receiving area for one point of a document, their vertical axis length is longer than their horizontal axis length. And, the gates of the first-to-third TFTs Q11 to Q13 are commonly connected to the first address line AD1 and the drain electrodes of the first-to third TFTs Q11 to Q13 are respectively connected to the first-to third data lines DL1 to DL3.
FIG. 3 is a cross sectional view taken on line A-A' of the first-to-third photodiodes PD11 to PD13 shown in FIG. 2. In FIG. 3, a CIS comprising a substrate 20 coated with an insulating film made of SiN is described. A plurality of metallic electrodes 22 made of chromium Cr are deposited on the insulating film 21. Light-receiving material layers 23 and transparent electrodes 24 are laminated on the plurality of metallic electrodes 22. The light receiving material layer 23 is made of amorphous silicon. A protecting film made of polyimide is deposited on the insulating film 21 and the transparent electrode 24.
As described above, since the conventional CIS is formed to be able to sense and process the gray level of characters, diagrams or drawings on a document, it can process documents having black and white information, but not a colored document.